Electrophotography is a commonly used process for printing images on a receiver such as paper or another media including glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (an “electrostatic latent image”).
After the electrostatic latent image is formed, charged toner particles are brought into the operative proximity of the photoreceptor and are attracted to the photoreceptor in such a manner as to convert or develop the electrostatic latent image into a visible image. It should be noted that the “visible image” includes images that may not be readily visible to the naked eye, depending on the composition of the toner particles (e.g. clear toner).
While simple photoreceptors consisting solely of photoconductive materials can be used, it is generally beneficial to use a composite photoreceptor. Typically, a photoreceptor comprises a support such as a polymer or metallic web or cylinder onto which is coated a conductive material such as nickel (this is not necessary when the support is electrically conducting). Coated onto the nickel is a photoconductive material that is capable of generating electron-hole pairs in the presence of an applied electrostatic field and actinic radiation such as obtained from a laser scanner or LED array. The photoconductive layer is overcoated with a charge transport layer that conducts only charge of a single polarity such as positively charged holes, while serving as a barrier to the charge carriers of the opposite polarity. Additional layers such as protective layers such as sol-gels, diamond-like carbon, or other ceramics, as well as release layers may also be present in a photoreceptive member.
The photoreceptor may be modeled in simple electrical circuit representation as a capacitor (Cp) and a single charging device, when operated in a constant voltage mode, may be represented by an ideal voltage source (Vc) and resistor (Rc). While the validity of such representation is not necessary to practice the present invention, it does help in understanding the problem. The charging time constant is given by τRC=RcCp. The residence time of the insulator within the charging zone is given by τres=W/U where W is the width of the charging zone and U is the photoreceptor surface speed. The charging device exponentially charges up the capacitor to Vc, achieving 63% of Vc after 1 charging time constant and 95% of Vc after 3 charging time constants. Typical for applications such as charging a photoreceptor to a uniform surface potential, the desire is to have τRC<τres so as to operate at or near the saturation level of the charging curve, minimizing sensitivity to variations in the charging process and thereby maximizing surface potential uniformity at a specified level. This presents challenges as printers strive for faster printing speeds and smaller size whereas, as previously discussed, the residence time varies directly with charging device width and inversely in printing speed. It becomes difficult to have a single charging device deliver the range of surface potentials required and have a short enough charging time constant to provide good surface potential uniformity.
When operating the charging device in a constant current mode in which the current being held constant is the current delivered to the surface of the photoreceptor, then the surface potential of the photoreceptor Vp may be calculated as
      V    p    =      σ                  (                  C          /          A                )            p      where σ is the charge per unit area (delivered to the surface) and (C/A)p is the photoreceptor capacitance per unit area. The charge per unit area σ (C/m2) may be calculated as
  σ  =            I      p              U      *      L      where Ip is the current delivered to the photoreceptor surface in μA, U is the machine speed in mm/sec, and L is the length of the corona electrode in mm. The photoreceptor capacitance per unit area may be calculated as
            (              C        A            )        p    =            ɛ      p              d      p      where ∈p is the permittivity (F/m) and dp is the thickness (m) of the photoreceptor.
It is well known in the art to use a conductive shell partially surrounding a corona electrode, particularly for DC coronas, so as to both reduce the corona electrode voltage required to initiate corona and to enhance the uniformity of the corona along the length of the corona electrode. However, a conductive shell maintained at a low potential relative to the corona electrode will attract a high percentage of the net current emanating from the corona electrode, reducing the efficiency of the charging device. It is also well known in the art to use a conductive grid, interposed between the corona electrode and the surface to be charged. The purpose of the conductive grid is to control both the level and uniformity of the surface potential on the photoreceptor. However, this conductive grid will also reduce the efficiency of the charging device.
A method of improved photoreceptor charging is described in U.S. Pat. No. 2,778,946. Disclosed is the utilization of an initial open wire DC charger having a conductive shell and used to deposit up to about 80% of the desired charge level, followed by a grid-controlled DC charger, also having a conductive shell, to provide the remaining 20% required to establish the final desired surface potential of the photoreceptor. However, this device drives the corona electrodes at a constant voltage and utilizes a grounded shell, so it will not have the charging efficiency required for present day printing speeds that are well in excess of 300 mm/sec.
A system for operating a corona charging device in a constant charging current mode is described in U.S. Pat. No. 5,079,669. The purpose of this device is to charge the surface of a photoreceptor to a uniform level and reduce sensitivity of the charging process to variations created by temperature, humidity, wear, and spacing between the charging device and the photoreceptor. In this disclosure and as shown in FIG. 1, the current Is flowing through shell 34 is summed with the current Ip flowing to the photoreceptor using current summing node 54 and employs resistor 84 to do so, as shown in FIG. 1. The voltage on shell 34 is determined by the product of Ip and resistor 84 and is of a low voltage on the order of 5V given Ip values on the order of 50 μA and resistor values on the order of 100 kΩ as provided in the disclosure. This shell voltage is about 1000× lower than the wire voltage of 5 kV. Furthermore, the shell voltage is also the negative input to operational amplifier 64 and as such would typically be in the range of 0V to 5V. As before, the low voltage shell will not have the charging efficiency required for present day printing speeds that are well in excess of 300 mm/sec.
A method for improving the charging efficiency of a corona charging device is described in U.S. Pat. No. 3,769,506. In this invention the charging output is enhanced by raising the potential of the shell to a voltage level of the same order of magnitude as the corona wire, either by connecting the shell to a second high voltage source or by connecting the shell to ground via a high resistance element. For example, using a shell resistance value of 10 MΩ, a shell current flow of 10 μA/in, and a corona length of 10 inches, results in a shell voltage of 1 kV, well within an order of magnitude of a corona wire voltage range of 3.5 to 8 kV. This provides greater charging and power efficiency for the device. However, this device is operated in a constant voltage mode for rapidly charging the surface of a photoreceptor. Further, this device is shown as a single stage charging device without any grid to control the electric field and charge flow between the corona wire and the surface of the photoreceptor. This will result in highly non-uniform charging due to the non-uniform electric field between the corona wire and the photoreceptor surface.
A method for charging the surface of a photoreceptor utilizing a shell electrode connected to a high voltage DC power supply is described in U.S. Pat. No. 4,086,650. In this reference the shell electrode is biased to either a positive or negative voltage or is grounded depending upon the surface potential desired for the photoconductive surface to be charged. When used in combination with a dielectric coated corona wire biased to a high AC voltage, the uniformity of the surface potential on the photoreceptor is improved. However, the high voltage AC power supply adds cost and power consumption to the device operation.
A method for low sensitivity corona charging of a photoreceptor surface is described in U.S. Pat. No. 4,245,272. A boost and trim strategy is described whereby the first corona charging stage is used to overcharge the photoreceptor surface above the desired potential level and the subsequent stage(s) are used to reduce back to the desired potential level. This technique utilizes a DC-biased AC voltage source to drive the corona electrodes. As stated above, the high voltage AC power supply adds cost and power consumption to the device operation.
A method for improved photoreceptor charging uniformity is described in U.S. Pat. No. 6,134,095. The use of aperiodic grids in conjunction with a DC-offset AC corona charging device is disclosed whereby a significant improvement in charging uniformity is achieved relative to a grid having a uniform grid element spacing. The aperiodic grid is described as having a grid transparency (percent area opening) that varies significantly from the entrance to the exit of the charging region beneath the charging device. However, this disclosure utilizes DC-offset AC corona charging, necessitating an expensive power supply and creating significant electromagnetic emissions.
There is a continuing need, therefore, for uniform non-contact charging of a photoreceptor surface in an efficient, low cost manner.